The present invention relates to a hologram lens, and particularly to an aberration-reduced spherical hologram lens which is free from coma aberration, and which can be suitably used as an optical head of an optical memory such as optical disc device or as an objective lens of scanning optics in a laser printer.
It is desired that the objective lens employed in the optical head of an optical disc device and the like is as small as possible, light in weight, and is inexpensive. However, as the numerical aperture NA of the objective lens increases, there develops lens aberration. At present, the aberration has been corrected by combining several single lenses. For instance, the objective lens employed in the optical head of an optical disc device consists of an objective lens having a numerical aperture NA of about 0.5 on the side of the optical disc and an objective lens having a numerical aperture NA of about 0.2 on the side of the light source. Usually, the former lens assembly consists of three to four lenses assembly and the latter lens consists of two lenses.
Therefore, a limitation is imposed on reducing the weight of objective lens or on reducing the manufacturing cost thereof. This makes it difficult to enhance the tracking performance of pit trains, that is carried out at the time of reproducing or recording signals of the optical disc, and makes it difficult to enhance the performance of an actuator which effects automatic focusing, to reduce the size thereof, and to reduce the manufacturing cost thereof. This fact also hinders the whole optical head from being constructed in a small size and in a reduced weight.
Under such circumstances, it is an assignment to reduce the size and weight of the objective lens, and some proposals have been offered. Among them, attention has been given to a hologram lens utilizing the art of holography owing to its features such as reduced weight, cheap cost, and easy replication. According to the holography in principle, a given wave front is recorded in the form of an interference pattern which can be reproduced. Therefore, a hologram lens having a numerical aperture of greater, for instance, than 0.5, i.e., having a large divergence angle of beams, can be obtained easily, and imaging properties like those of ordinary optical lenses can also be obtained. With the hologram lens, however, the wave front must be reproduced under quite the same conditions as those of the recording operation. If the conditions are changed even by slightest amounts, there develops a serious wave front aberration. Therefore, the optical axes must be postioned maintaining a very high precision, hindering the hologram lens from being placed in practical use. Summary of the Invention
The object of the present invention is to provide a hologram lens which precludes the above-mentioned defects, and which produces aberration in reduced amounts even when the conditions are different between the recording operation and the reproducing operation.
In the hologram lens, coma aberration is dominant among five types of Seidel aberrations. This stems from the fact that a hologram plate has a flat surface, and sine conditions are not satisfied. According to the present invention, therefore, the above object is accomplished by providing a spherical hologram lens in which the center of a hologram surface is brought into agreement with the focal point of a lens, the hologram has such a radius that the curvature thereof is brought into agreement with the focal distance of the lens, and the sine condition is satisfied to remove coma aberration.
The hologram lens referred to in the present invention includes a grating lens which records an interference pattern by an electron beam, a zone plate, and replica lenses thereof, in addition to a so-called hologram lens which optically records an interference pattern by using two beams, i.e., by using an object beam and a reference beam.